(a) Field of the Invention
The present invention relates to improvements in valves and more particularly to improvements in bonnetless valves for high pressure applications. The present invention also relates to a method of servicing the aforesaid valves.
(b) Description of Prior Art
High pressure globe valves half inch to four inch, for steam and high pressure processing in industry, power, ship building, etc. are made in two different styles so as to eliminate any possibility of body-bonnet leakage. Gasketed joints between body and bonnet which may develop leaks are not acceptable. This problem is solved in two well known ways. The first one is to provide a screwed-in bonnet which is thereafter seal welded. Examples of such valves include: The Rockwell-Edward forged steel Univalve*; R.P&C Valve, Division of White Consolidated Industries, Inc. Class 1500 and 2500Y Pattern globe valves; Henry Vogt Machine Co. valves of the 1700-255 pounds service class; valves manufactured by Bont, and Serseg in France, include non-turning stems. Valves of the type featuring a bolted bonnet which is thereafter seal welded are also available. Examples of such valves include the Babcock NICO 3000 Type 111-234 globe valves with integral backseat on the bonnet. All such valves are difficult to disassemble for servicing, as the seal weld has to be cut, which is a difficult operation to perform in for example a nuclear power station, in a radioactive environment or in inaccessible locations. Furthermore, the problem is compounded following servicing of the valve since the joint has to be rewelded, which is costly and generally difficult to perform in the field. The solution to this problem has been awaited for a number of years. FNT *Trade Mark of Rockwell Flow Control Division of Rockwell-Edward
The aforementioned second way is to provide valves of the bonnetless type without any weld. Valves of this type are manufactured in North America by the applicant; Yarway Corporation under the trade name Weldbond* and Dresser Industries Inc. Only applicant's valves however are made from a single body forging without any weld. In the case of Dresser, the valve body comprises two pieces which are then welded together to become a single body. In the case of Yarway, the yoke is screwed on the body and thereafter welded so that the unit also becomes bonnet-less like applicant's. German manufacturers--Babcock and Sempell--and French manufacturer Sapag offer bonnetless valve, however, a substantial servicing problem exists in respect of these valves. This concerns removal of internal parts in the line due to screwed in backseats. In all existing designs, the backseat has to be screwed in place in the body of the valve and thereafter covered up with packing rings. These packing rings are pressed into the valve body packing chamber in order to provide a seal between the valve stem and the valve body. To disassemble the valve, one has to remove each individual packing ring first, which although is less costly and difficult than removing welds and rewelding as discussed above in respect to body bonnet welded joint type valves, it is still a very time consuming and therefore expensive operation. Once the packing rings are removed, the backseat is removed with a special tool and servicing may then commence. The assembly of such valves is relatively easy. It has been noted that Babcock and Sempell offer bonnetless valves only for those applications where a backseat is not required and offer a bolted bonnet, seal welded valve for applications requiring a backseat. FNT *Trade Mark of Yarway Corporation